Glass in thin sheet form, termed microsheet in the glass industry, i.e., glass sheeting having thickness dimensions customarily ranging between about 0.002"-0.025", has been produced for many years. For example, Corning Glass Works, Corning, N.Y. has marketed Code 0211 glass in thin sheet form for such diverse applications as cover slips for microscope slides used in laboratories, as thin and thick film substrates for use in magnetic film memory devices, liquid crystal displays, and electroluminescent devices, and in the form of larger pieces have been laminated to transparent plastic to provide windows of high durability and excellent resistance to abrasion and impacts. Such glass has long been prepared in individual sheet form of various dimensions and, more recently, as continuous sheeting which is rolled onto a spool.
One important technical application for microsheet consists of a protective covering for silicon solar cells utilized in the generation of electrical energy. The highly transparent sheeting is bonded to the solar cell by means of a transparent polymeric adhesive and protects the delicate cell from dirt, from mechanical abrasion and impacts, and from the effects of weathering, without significantly affecting the amount of solar energy which impinges upon the cell. Furthermore, because the microsheet is so thin [conventionally about 0.006" (0.152 mm) or less], very little weight is added by the protective glass sheet.
Manufacturers of solar cells have found that in certain energetically-hostile environments, e.g., satellites in earth orbit or elsewhere in space, the efficiency of the solar cell decreases with time because of the darkening of the protective glass, this darkening being generically termed solarization. High energy particles and photons cause damage in the glass at the atomic level, thereby decreasing the energy which reaches the silicon cell. A description of solarization and similar phenomena is provided in Glass Structure by Spectroscopy, J. Wong and C. A. Angell, Marcel Dekker, Inc., 1976, in chapter 6, section 6.5, "Irradiation--Induced Optical Phenomena."
One known remedy for solarization of glass comprises the inclusion of cerium, normally in the form of cerium oxide, in the glass composition. Wong and Angell explain a mechanism through which cerium may protect the glass from darkening.
The forming of glass microsheet is a very delicate and unforgiving process. Thus, the forming properties of the glass require extremely stringent control; foremost among those critical chracteristics being the liquidus temperature of the glass or, more accurately, the maximum temperature of devitrification. Therefore, any significant increase in the tendency of the glass to develop crystals at its forming viscosity (in the vicinity of 100,000 poises) will lead to an unacceptable product, or even failure of the manufacturing process.
The optical transmission properties of the glass must be stringently controlled. Thus, the glass must be highly absorbing of radiation at wavelengths below about 350 nm so that the polymeric adhesive material is protected from degradation. On the other hand, the glass must be highly transparent to radiations having wavelengths longer than about 370 nm so that solar energy will be transmitted to the underlying solar cell. That latter requirement has been translated into a specification demanding a transmittance at a thickness of 0.006" greater than 50% at a wavelength of 370 nm. That feature effectively defines an "ultraviolet cutoff."
Corning Code 0211 glass has the following approximate composition, expressed in terms of weight percent on the oxide basis, of
______________________________________ SiO.sub.2 64.3 K.sub.2 O 6.65 Al.sub.2 O.sub.3 2.25 ZnO 7.0 B.sub.2 O.sub.3 9.3 TiO.sub.2 3.11 Na.sub.2 O 7.15 Sb.sub.2 O.sub.3 0.25 ______________________________________
and exhibits the following physical properties
______________________________________ Softening Point 720.degree. C. Annealing Point 550.degree. C. Strain Point 508.degree. C. Coefficient of Thermal Expansion (0.degree.-300.degree. C.) 74 .times. 10.sup.-7 /.degree.C. 24 Hour Liquidus 890.degree. C. ______________________________________
In view of the desired thinness of the microsheet (no greater than about 0.006"), it was calculated that at least 4% by weight CeO.sub.2 and, preferably, a minimum of 5% CeO.sub.2 would be required to provide the necessary resistance to solarization. The simple addition of CeO.sub.2 to the Code 0211 glass composition dramatically raised the liquidus temperature thereof and did not yield the necessary ultraviolet radiation transmittance at a wavelength of 370 nm.
Accordingly, the primary objective of the present invention was to develop glass compositions exhibiting melting and forming properties rendering them capable of being drawn into microsheet utilizing melting and forming apparatus and technology conventional in the drawing of microsheet, particularly an internal liquidus below 1050.degree. C., preferably below 1000.degree. C., and most preferably below 950.degree. C., but which would demonstrate excellent resistance to solarization and manifest an effective ultraviolet cutoff at a wavelength of 370 nm.